Interposer to regulate current for wafer test tooling

ABSTRACT

An interposer is described to regulate the current in wafer test tooling. In one example, the interposer includes a first connection pad to couple to automated test equipment and a second connection pad to couple to a device under test. The interposer further includes an overcurrent limit circuit to connect the first and second connection pads and to disconnect the first and second connection pads when the current between the first and second connection pads is over a predetermined amount.

BACKGROUND

After a semiconductor or silicon wafer is produced it is processedthrough a variety of tests to ensure that it works as intended. One ofthe test tooling units places contacts onto the connection bumps of eachdie to test bump integrity and also to test for shorts and grounds.Currents are applied to each connection bump of each die. Among thetests are measurements of currents that occur from applied voltages andcurrents. If there is a failure at one or more of the connection bumpsof a die during a test, however, there may be an over current conditionat the connection bump. This may propagate through the test tool anddamage the test tool. Wafer test tooling, for example, a Sort InterfaceUnit (SIU) has no ability to protect itself from individual wafer bumpor probe level failures while testing silicon wafers. As a result, theservice life of the test tooling is reduced.

Damage to the tool can be mitigated by modifying the test program thatcontrols the automated test equipment (ATE testers). However, thisapproach does not correct for the failure of individual probes and istoo slow to protect against overcurrent power supply clamping events.Damage can also be mitigated by increasing the current carryingcapabilities of the wafer probes. However, the current carrying abilityof a probe is determined by the physical dimensions of the probe so thatas the connection bumps become closer together with future chipgenerations, the size of the probes must also be reduced. Larger, highercurrent probes cannot be used when the connection points are very closetogether. The effects of over current can also be mitigated usingmodification of the printed circuit boards of the test tooling. Howeversuch circuitry cannot address failures at a particular probe because theconnections to multiple probes are combined together at the printedcircuit board. In addition, due to the long leads from each probe to theprinted circuit board the response time is too slow for overcurrentevents.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings inwhich like reference numerals refer to similar elements.

FIG. 1 is cross-sectional diagram or a wafer test tool having a currentregulation interposer according to an embodiment of the invention.

FIG. 2 is a functional block diagram of an example of a wafer test toolhaving a current regulation interposer according to an embodiment of theinvention.

FIG. 3 is a circuit diagram of a current regulation circuit suitable foruse in a current regulation interposer according to an embodiment of theinvention.

FIG. 4 is a cross-sectional diagram of a current regulation interposeraccording to an embodiment of the invention.

FIG. 5 is functional block diagram of a current regulation circuit andsignal pathways suitable for use in a current regulation interposeraccording to an embodiment of the invention.

DETAILED DESCRIPTION

A current limiting circuit block architecture can be implemented into asilicon based space transformer in a wafer or die test tool. The spacetransformer serves to protect the wafer test tooling from over-currentevents induced by performing functional tests on connection points of adie.

A silicon substrate may be added to test tooling that contains activecircuitry for probe over current protection. The substrate may also havethrough silicon vias (TSV) for system interconnect. FIG. 1 shows howsuch a substrate may be used in the context of a portion 100 of atypical SIU (Sort Interface Unit). The DUT (device under test) 102 is adie that may or may not be part of a larger wafer. In some systems thewafer is placed on a turntable and the wafer is rotated to place eachdie under a test head. One or more dies may be tested at the same time.The die may be semiconductor, micromechanical, optical or any other typeof die. The die is shown as having set of connection balls 104 thatallow for connection with the test equipment and at least some of whichwill allow for connection to other circuitry after the die is packaged.

The test system 100 has a set of wafer probes to make contact with theconnection balls of the die. The wafer probes apply currents andvoltages to the connection balls and measure the results. The probes aresusceptible to damage if there is a flaw in the die that allows thecurrent or voltage to exceed the capacity of the probe. The probes aremade of various metal alloys and can be thought of as a very thin wire.Physical size limitations exist due to the high density or fine pitch ofthe die connection pads and balls. The high density/fine pitch meansthat the balls are very close together, so that the amount of physicalspace available for each probe is limited.

The test system also has a PCB (printed circuit board) 110 that allowsfor probes to be coupled to cables or circuits or other devices. The PCBis coupled through a cable 115 to an electronic automated tester 116.The automated tester generates electrical signals in an appropriatesequence and with appropriate values. These are sent through the cable115 to the PCB and from the PCB to the probes. Sensed signals receivedthrough the probes and other sensors (not shown) travel in the oppositedirection.

The probes 106 are connected to the PCB 110 through a ceramic, organic,or ceramic organic package 108, also referred to as a space transformerthat is used to translate the system printed circuit board electricalconnection pitch to wafer bump pitch. From the space transformer 108,the signals are coupled through a space transform to PCB interposer 112,a

For structural purposes, FIG. 1 also shows top side stiffening hardware114 to carry the PCB interposers and probes as well as bottom sidehardware 116 coupled together with clamping screws 118 in four or morelocations. Further structural and convenience hardware (not shown) maybe included, depending on the particular application for the testsystem. There may also be additional electrical and electroniccomponents. A set of back side capacitors 120 is shown as one example.The back side capacitors are attached to the back side of the spacetransformer in an open area of the back side of the transformer. Thecapacitors serve to avoid static discharge, power supply decoupling, andother problems. Other discrete or integrated components may be usedinstead of or in addition to the capacitors in the same or in differentpositions.

While the wafer contact probes 106 are typically attached directly tothe space transformer 108. In the example of FIG. 1, an additionalinterposer 122 is placed between the probe contacts and the spacetransformer. The silicon interposer is equipped with active circuits toprotect the contract probes from an overcurrent and to isolate the restof the system from the overcurrent. In one example, the interposer hascurrent regulation circuitry. Electrical redistribution layers, analogMOSFETs (Metal Oxide Semiconductor Field Effect Transistor), and controllogic may be included for current regulation circuit operation. Thelayer may also include communication blocks for external control, andTSVs (Through Silicon Vias) for interconnection to the rest of thesystem platform.

In one example, using this current regulation interposer 122, when adefective die requests a current load larger than the capability of theindividual probe, the regulation circuit will detect this event. Usingthe active circuits, the circuit may then limit the current draw to apredefined level suitable for reliable system operation. At the sametime communications may be sent to the ATE (Automated Test Equipment) toidentify that an event has occurred. Additional and different functionsmay be provided by additional active and passive circuitry in thecurrent regulation interposer.

The position of the interposer greatly improves its ability to protectthe test equipment. An overcurrent condition is detected almostimmediately because the interposer is directly connected to each probe.The condition can also be isolated to the affected probes withoutaffecting other probes. In addition, if the current is between twoprobes, then the connection can be broken to stop the current flow. Inthe space transformer and the PCB, the connections for some of theprobes are connected together. This is particularly true for powerconnections. As an example, typically many of the connection balls onthe die are for power supply to the die. Connecting all of thecorresponding probe contacts to a single power source greatly simplifiesthe test equipment. However, because all of the probes are coupledtogether, many probes may be damaged by an overcurrent in the powersupply connections. At the current regulation interposer, however, eachcontact probe is independently coupled to the interposer. As shown theinterposer 122 also has a grid of connection to the space transformer108. These connections may be designed to connect to the spacetransformer in exactly the same position that the contact probes wouldotherwise connect to the space transformer.

FIG. 2 is a diagram of with simplified circuit elements of currentregulation interposer 122 in more detail coupled to the other componentsof the tester 100 of FIG. 1 for context. The DUT 102 is coupled throughits connection pads to probes 106. The probes are represented as ahaving resistance 202 at the connection to the DUT and also having aninductance-resistance (LR) loop as an inherent property of theconductive metal probe. The probes are coupled to the silicon interposer122 which is coupled to the space transformer 108 which is coupled tothe PCB interposer 112 which is coupled to the PCB 110 which is coupledto the ATE tester 116. The connections are shown as direct connectionsin this diagram for purposes of explanation. However, the specificnature of the connection will depend upon the nature of the physicalconstruction of the components and may be by wire, cable, direct solderor ball connection or by a variety of other connection and couplingtechnologies.

The ATE tester 116 has two connection sections as shown. More sectionsmay also be used to support additional functions. One section 212couples to the DUT 102 to drive the DUT through its various modes. Ithas a power supply 216, a return ground 220, and an I/O (Input/Output)data connection section 218 which is typically bidirectional to drivesignals onto the DUT and also to receive signals from the DUT. The poweris supplied through the current limiting interposer 122 through a powerline 222. The power line is coupled to an appropriate probe 106 tosupply power to the DUT during test. The ground 220 is similarly coupledthrough the interposer using a ground line 226 to an appropriate probe106. The power and ground lines are coupled to the probes usingconductive lines through the silicon of the interposer. This allowsadditional current regulation circuitry to be connected to the lines.The I/O signals on the contrary are coupled through TSVs 224 directly toappropriate probes. This isolates the I/O signals from any effects ofthe silicon interposer 122 and its circuitry. As indicated in thediagram of FIG. 2, the space transformer 108 changes the positions ofthe connections between the PCB and the probes. However the currentregulation interposer may be designed so that there is no change inpitch or position of the connectors.

The ATE tester has a second connection section 214 for the currentregulation circuitry of the current regulation interposer. In thisexample, the ATE drives and controls the current regulation interposer.A power line 232 and a ground line 234 provide power to control acontrol block cell 240 of the interposer. An I/O line 236 allows data tobe transferred to and from the control block cell and a clock line 238allows synchronous data and test signals to be communicated between thetester and the control block cell. The control block cell has a currentsensor 244 that detects the current on the power line to the DUT acrossa resistor 246. The control block cell also has a gate/source voltagecontroller 242 coupled to a MOSFET 248. The MOSFET has its source anddrain coupled across the power line to the DUT. As shown in the diagramsome or part of the lines through the interposer may be routed throughTSVs 250 for isolation from the other circuitry and to reduce lineimpedance and capacitance.

Additional control block cells 252 may be used for ground signals andfor other power signals. Each control block cell 252 includes a voltagecontroller 252 coupled to a respective MOSFET 256 and a current detector254. Each control block cell is also coupled to the ATE tester. Whilepower supply connections 232, 234, may be shared the I/O 236, and clocksignals 238 may each be dedicated to a specific control block cell.

In this example, silicon wafer processing techniques may be used topattern the MOSFET device 248 of the interposer 122. The MOSFET may haveelectrical specifications that allow the source to drain current,whether p-FET or n-FET, to flow in the linear region of the ID (DrainCurrent) vs. VDS (Drain to Source Voltage) curve during normaloperation. When the VDS exceeds a threshold defined by the waferprobe/solder ball component specifications, the ID is then saturated.Biasing the gate voltage by the control block cell 240 makes the MOSFETa current limiter or regulator circuit. In addition, the MOSFET devicemay be designed to have a low RDSon (Drain to source on resistance)performance. This reduces the impact on the power distribution path 222,226, both its resistance and its inductance in the linear region ofoperation.

The silicon interposer may be fabricated as a die on a wafer with theMOSFETs embedded and with the through silicon vias 250 on the backsideof the die. The backside of the die will may then be attached to amultilayer substrate which provides the electrical connections for thesystem main board PCB 110. The front side of the die may be patternedand deposited to add redistribution layers to the top surface. Theseredistribution layers may be used to design feed lines to connect thecurrent regulation MOSFETs and I/Os to the pads specific to the DUT.After the design and manufacturing of the interposer has been completed,manufacturing and assembly steps may be used to attach the waferprobe/solder balls to the interposer.

FIG. 3 shows an alternative circuit design for current regulation in theinterposer. This circuit is particularly useful for an implementation inwhich a single power MOSFET cannot be created to switch off in a regionof operation suitable to the application voltage and currentrequirement. However, it may be used to provide greater or more precisecontrol in other situations. Additional logic blocks are used to ensurethat the circuit effectively regulates the current. The I/O signal 302bypasses the circuit and is connected between the space transformer 108and an appropriate probe 102. This may be done, in part, using TSVs. Apower or ground line 304 is also coupled between the space transformer108 and a probe pin 106. The power line is coupled to the currentregulation circuitry that is controlled by the ATE tester through linesprovided through the space transformer.

Circuit logic blocks are added to aid in the voltage reference settingswhich will control the VG (Gate Voltage) gate bias of the MOSFET. Thisin turn will turn the power MOSFET on or off. The current through thepower line across a resistor is detected by a buffer 308. The output ofthe buffer is a voltage proportional to the sensed current and isapplied to a comparator 310. The comparator compares the buffer voltageto a reference voltage 312 that can be designed internally to theinterposer or may be provided by the ATE tester. A digital latch 314with reset operation 318 controlled by the tester receives thecomparator output 316 as a set signal.

When the buffer output indicates an overcurrent condition, this will bedetermined when the comparator determines that the buffer voltageexceeds the threshold of the reference voltage. The comparator will thensend a high set signal 316 to the RS (reset) latch. The latch then sendsa high signal at the latch 320 to the MOSFET device 322 on the powerline breaking the power line. The power line is then isolated betweenthe probe and the space transformer. As a result, the flow of currentthrough the interposer 122 is stopped for that pin and the effects ofthe overcurrent are limited to the die, the pin and the interposer.

The latch signal may also be used to send a signal to the test programof the ATE, through, for example an I/O port 318 for error detection.The test program after correcting the cause of the overcurrent may thenset the MOSFET back into the “on” state (for a normally on p-FET). Sincethese logic components are small in area compared to the large areaneeded for the power FET they can be considered negligible from a layoutconstraint standpoint. The additional inputs used in this example arethe programmable reference voltage 312 to compare against the voltagedrop over the resistor. In addition a reset line 318 is routed.

The cell block, the logic components, and the power MOSFETs can berepeated over the silicon interposer. To simplify the connections, cellto cell communication can be created like a clock tree network to allowfewer lines to be used for communication from the external hardware toeach cell.

FIG. 4 is an alternative cross-sectional diagram of the currentregulation interposer 122 described above. The interposer connects apower distribution plane 401 coupled through the space transformer 108to a logic device power rail 404 of the DUT 102. Power lines 406 coupleto pads 410 on the back side of the interposer 122. The power is carriedthrough the interposer through vias 414 to a contact 412 at the desiredhorizontal layer, such as a metal layer, of the interposer. A currentregulation circuit 416 is coupled to the power line through thehorizontal layer to interrupt the connection when there is anovercurrent condition. The power line continues across to a second pad418 at the same layer that couples the power into a second via 422 toanother connection pad 424. The line is translated horizontally at thelayer of this pad 424 to another via 426 and through the last via 426 toa final pad 428 on the interposer exterior.

The number of pads and vias may be adapted to suit any particularimplementation to translate the position of the pads or to route signalsefficiently through the interposer. In addition to translating thepositions of the inputs and providing access to the current regulationcircuit, in the illustrated example, the layers and vias perform anotherfunction. As can be seen the first via 414 coupled to the powerdistribution plane is very large. This physical size can allow it tocarry a large current. The second 418 and third 426 vias, however, aremuch smaller. The conductive layers may be used to distribute power fromthe one large via 414 to many smaller vias 422. As shown the finalconnection pad 428 on the front side of the interposer connects to aprobe. The system may use a single power connection pad 410 todistribute power to many probes 408. With the distribution and theovercurrent regulation both being performed in the interposer, thewiring may be simplified and still any overcurrent conditions can belimited to the particular probes involved. If the overcurrent circuitssensed current on multiple pins, then the overcurrent might bepropagated from one probe to another through the shared connection. Inaddition, it might not be possible to identify precisely which probesare affected.

FIG. 5 is a cross-sectional diagram that may be applied to theinterposer 122 shown in FIG. 4. The diagram is designed to show variousfeatures and functions that are in addition to those shown in FIG. 5 toaid in further understanding the examples described above. Theinterposer 502 as shown in this diagram has an I/0 input 504 coupled torespective probes. The I/O inputs each are conducted through dedicatedI/0 TSVs 506 and are coupled through an I/O output through variousintermediate devices to the ATE tester.

Similarly a clamp input 510 from a probe coupled to a power connectionpad on the DUT is coupled through a clamp circuit 512 to a large TSV514. The clamp circuit may use a current regulation circuit as describedabove or any other type of circuit to protect other components fromovercurrent at the probe. The placement of the clamp circuit between theclamp input 510 and the large TSV 514, which may be share with otherprobes, isolates the overcurrent condition to the affected clamp input.The clamp output is coupled to the power supply for the DUT which istypically the ATE tester.

The interposer also includes clamp circuit control logic 518 and controlblock cells 520. These devices may be duplicated for each probe orshared with a subset of the total number of probes. As described abovethe clamp circuit control logic may be used to set limits, thresholdsand reference levels for the clamp circuit. The claim circuit cuts theconnection between the clamp input and output when the overcurrentcondition is detected. The control block cell communicates with controlinterfaces from the detector to report conditions and reset the controllogic based on commands from the tester. The clamp circuit control logicand the control block cells are optional components and may beimplemented in any of a variety of different ways or not at all.

It is to be appreciated that a lesser or more equipped system than theexamples described above may be preferred for certain implementations.Therefore, the configuration of the exemplary systems and circuits mayvary from implementation to implementation depending upon numerousfactors, such as price constraints, performance requirements,technological improvements, or other circumstances.

Embodiments may be implemented as any or a combination of: one or moremicrochips or integrated circuits interconnected using a motherboard,hardwired logic, software stored by a memory device and executed by amicroprocessor, firmware, an application specific integrated circuit(ASIC), and/or a field programmable gate array (FPGA). The term “logic”may include, by way of example, software or hardware and/or combinationsof software and hardware.

References to “one embodiment”, “an embodiment”, “example embodiment”,“various embodiments”, etc., indicate that the embodiment(s) of theinvention so described may include particular features, structures, orcharacteristics, but not every embodiment necessarily includes theparticular features, structures, or characteristics. Further, someembodiments may have some, all, or none of the features described forother embodiments.

In the following description and claims, the term “coupled” along withits derivatives, may be used. “Coupled” is used to indicate that two ormore elements co-operate or interact with each other, but they may ormay not have intervening physical or electrical components between them.

As used in the claims, unless otherwise specified, the use of theordinal adjectives “first”, “second”, “third”, etc., to describe acommon element, merely indicate that different instances of likeelements are being referred to, and are not intended to imply that theelements so described must be in a given sequence, either temporally,spatially, in ranking, or in any other manner.

The drawings and the forgoing description give examples of embodiments.Those skilled in the art will appreciate that one or more of thedescribed elements may well be combined into a single functionalelement. Alternatively, certain elements may be split into multiplefunctional elements. Elements from one embodiment may be added toanother embodiment. For example, orders of processes described hereinmay be changed and are not limited to the manner described herein.Moreover, the actions of any flow diagram need not be implemented in theorder shown; nor do all of the acts necessarily need to be performed.Also, those acts that are not dependent on other acts may be performedin parallel with the other acts. The scope of embodiments is by no meanslimited by these specific examples. Numerous variations, whetherexplicitly given in the specification or not, such as differences instructure, dimension, and use of material, are possible. The scope ofembodiments is at least as broad as given by the following claims.

The following examples pertain to further embodiments. The variousfeatures of the different embodiments may be variously combined withsome features included and others excluded to suit a variety ofdifferent applications. Some embodiments pertain to an apparatuscomprising: a first connection pad to couple to automated testequipment; a second connection pad to couple to a device under test; andan overcurrent limit circuit to connect the first and second connectionpads and to disconnect the first and second connection pads when thecurrent between the first and second connection pads is over apredetermined amount.

Additional features include: the second connection pad is to connect apower supply to a probe to contact a connector of the device under test;the overcurrent limit circuit comprises a semiconductor switch toconnect and disconnect the first and second connection pads in responseto a sensed current; the switch is a transistor and the sensed currentis applied to a gate of the transistor; the switch is a transistor, theapparatus further comprising a current sensor to compare the current toa threshold and to apply a switching voltage to a gate of the transistorif the sensed current exceeds the threshold; and a control interface tothe automated test equipment to receive commands to reset the transistorand to control the threshold.

Additional features include: a multilayer silicon substrate to carry thefirst and second connection pads on opposite sides and the overcurrentlimit circuit; a first through silicon via to connect the firstconnection pad to the overcurrent limit circuit and a second throughsilicon via to connect the overcurrent limit circuit to the secondconnection pad; a metal layer to translate the position of the secondconnection pad from the overcurrent limit circuit; a probe coupled tothe second connection pad to contact an electrical contact point of thedevice under test; wherein the probe is directly connected to the secondconnection pad; and a plurality of overcurrent limit circuits coupled tothe first connection pad and a plurality of second connection pads, eachconnected to an overcurrent limit circuit so that each overcurrent limitcircuit isolates the respective second connection pad from the firstconnection pad in the event of an overcurrent at the respective secondconnection pad.

Other embodiments pertain to a multilayer silicon substrate comprising:a first connection pad to couple to automated test equipment; a secondconnection pad to couple to a device under test; a first via through thesubstrate to connect to the first connection pad; a second via throughthe substrate to connect to the second connection pad; metal layerwithin the substrate to connect the first and second vias; and anovercurrent limit circuit formed in the substrate to connect the firstand second connection pads and to disconnect the first and secondconnection pads when the current between the first and second connectionpads is over a predetermined amount.

Additional features may include: the second connection pad is to connecta power supply to a probe to contact a connector of the device undertest; the overcurrent limit circuit comprises a semiconductor switch toconnect and disconnect the first and second connection pads in responseto a sensed current; and the switch is a transistor, the apparatusfurther comprising a current sensor to compare the current to athreshold and to apply a switching voltage to a gate of the transistorif the sensed current exceeds the threshold.

Other embodiments pertain to means for coupling a first connection padto automated test equipment; means for coupling a second connection padto a device under test; and means to connect the first and secondconnection pads and to disconnect the first and second connection padswhen the current between the first and second connection pads is over apredetermined amount.

Additional features include: means to compare the current to a thresholdand to apply a switching voltage to the means to connect and disconnectif the sensed current exceeds the threshold; means for resetting themeans for connecting and disconnecting and means to control thethreshold; and means to connect the first connection pad to a pluralityof overcurrent limit circuits and means to connect each overcurrentlimit circuit each to one of a plurality of second connection pads.

1. An apparatus comprising: a first connection pad to couple to a spacetransformer of automated test equipment, the space transformertransforming a connector pitch from that of a printed circuit board tothat of a silicon substrate; a second connection pad to couple directlythrough probes to a device under test; and an overcurrent limit circuitbetween the first and second connection pads to electrically connect thefirst connection pad to the second connection pads and to electricallydisconnect the first and second connection pads when the current betweenthe first and second connection pads is over a predetermined amount. 2.The apparatus of claim 1, wherein the second connection pad is toconnect a power supply to a probe to contact a connector of the deviceunder test.
 3. The apparatus of claim 1, wherein the overcurrent limitcircuit comprises a semiconductor switch to connect and disconnect thefirst and second connection pads in response to a sensed current.
 4. Theapparatus of claim 3, wherein the switch is a transistor and the sensedcurrent is applied to a gate of the transistor.
 5. The apparatus ofclaim 3, wherein the switch is a transistor, the apparatus furthercomprising a current sensor to compare the current to a threshold and toapply a switching voltage to a gate of the transistor if the sensedcurrent exceeds the threshold.
 6. The apparatus of claim 5, furthercomprising a control interface to the automated test equipment toreceive commands to reset the transistor and to control the threshold.7. The apparatus of claim 1, comprising a multilayer silicon substrateto carry the first and second connection pads on opposite sides and theovercurrent limit circuit within the multilayers.
 8. The apparatus ofclaim 7, further comprising a first through silicon via to connect thefirst connection pad to the overcurrent limit circuit and a secondthrough silicon via to connect the overcurrent limit circuit to thesecond connection pad.
 9. The apparatus of claim 7, further comprising ametal layer to translate the position of the second connection pad fromthe overcurrent limit circuit.
 10. The apparatus of claim 1, furthercomprising a probe coupled to the second connection pad to contact anelectrical contact point of the device under test.
 11. The apparatus ofclaim 10, wherein the probe is directly connected to the secondconnection pad.
 12. The apparatus of claim 1, further comprising aplurality of overcurrent limit circuits coupled to the first connectionpad and a plurality of second connection pads, each connected to anovercurrent limit circuit so that each overcurrent limit circuitisolates the respective second connection pad from the first connectionpad in the event of an overcurrent at the respective second connectionpad.
 13. A multilayer silicon substrate comprising: a first connectionpad to couple to a space transformer of automated test equipment, thespace transformer transforming a connector pitch from that of a printedcircuit board to that of a silicon substrate; a second connection pad tocouple directly through probes to a device under test; a first viathrough the substrate to connect to the first connection pad; a secondvia through the substrate to connect to the second connection pad; ametal layer within the substrate to connect the first and second vias;and an overcurrent limit circuit formed in the substrate to connect thefirst and second connection pads and to disconnect the first and secondconnection pads when the current between the first and second connectionpads is over a predetermined amount.
 14. The substrate of claim 13,wherein the second connection pad is to connect a power supply to aprobe to contact a connector of the device under test.
 15. The substrateof claim 13, wherein the overcurrent limit circuit comprises asemiconductor switch to connect and disconnect the first and secondconnection pads in response to a sensed current.
 16. The substrate ofclaim 15, wherein the switch is a transistor, the apparatus furthercomprising a current sensor to compare the current to a threshold and toapply a switching voltage to a gate of the transistor if the sensedcurrent exceeds the threshold.
 17. An apparatus comprising: means forcoupling a first connection pad to a space transformer of automated testequipment, the space transform min a connector pitch from that of aprinted circuit board to that of a silicon substrate; means for couplinga second connection pad directly through probes to a device under test;and means to connect the first and second connection pads and todisconnect the first and second connection pads when the current betweenthe first and second connection pads is over a predetermined amount. 18.The apparatus of claim 17, further comprising means to compare thecurrent to a threshold and to apply a switching voltage to the means toconnect and disconnect if the current exceeds the threshold.
 19. Theapparatus of claim 18, further comprising means for resetting the meansfor connecting and disconnecting and means to control the threshold. 20.The apparatus of claim 17, further comprising means to connect the firstconnection pad to a plurality of overcurrent limit circuits and means toconnect each overcurrent limit circuit each to one of a plurality ofsecond connection pads.